In the simplest form such foil bearings, also referred to as slotted ring bearings, comprise a single foil strip of the width of the bearing and the length of the bearing circumference minus the clearance at the butt joint. The foil is inserted in a corresponding groove in a bearing sleeve, in a housing, or in a shaft or a bearing pin. The foil usually has a thickness of 1.5 mm. The depth of the groove corresponds approximately to half the thickness of the foil. These known bearings are subject to uniform wear over the entire circumference as the foil is not mounted in fixed position but may carry out relative movements. The materials most commonly used are polytetrafluoroethylene plastics, especially with fillers. Such foil bearings offer an inexpensive and convenient solution of many problems associated with bearings. The keeping of spare parts is greatly simplified as the bearing foils may be cut according to need from a piece of foil.
The sliding clearance provided corresponds approximately to that of sliding bearings having metallic sliding surfaces with which the tolerance in bearing clearance must be observed very strictly. While known metallic sliding bearings have poor dry running properties in general, these are improved in simple foil bearings. In known metallic sliding bearings the normal bearing clearance is less than 0.1 mm. The known foil bearings are not suited for extreme loads.
The bearing play of known radial-axial slide supports for pumps and the like among others must be determined in consideration of the operating temperatures. This applies above all to bearings which are lubricated by the medium being pumped, for example in the chemical industry where also contaminations and the like may be contained in the lubricating medium. Often acceptable lifetimes of sliding bearings are achieved only by long-lasting tests, otherwise frequent exchange of bearings will have to be put up with. Under any circumstances, the pairing of materials of different hardness for the bearing sleeve and the bearing pin must be kept in mind, as the medium to be pumped restricts the choice of materials. This renders the association of the individual materials very difficult.
Another disadvantage of sliding bearings which are lubricated by the medium being pumped is that they must not be let run dry. It still belongs to the state of the art of pumps that their bearings must not be started up dry. It is generally known that a great number of pumps get defective bearings within seconds because this instruction is disregarded. The disassembly and exchange of the bearings involve high costs and the preventive measures to be provided cost a lot of time and money. The operational safety is endangered.
Also known are shaped foil bearings in which overlapping metal foil sections are spaced around the interior surface of a bushing in continuous slots (U.S. Pat. No. 3,957,317). These may not be utilized in pumps for treatment of chemicals, in particular not as they wear quickly.